The present invention relates to a liquid enclosing type vibration isolating mount used mainly in supporting a vibration generating body such as an automobile engine.
There has been known a liquid enclosing type vibration isolating mount as a vibration isolating mount supporting a vibration generating body such as an automobile engine to prevent the vibration from transmitting to a vehicle body. An attenuating function for a low frequency vibration is assigned to a liquid enclosed in a volume variable liquid chamber while a vibration isolating function for a high frequency vibration is assigned to a vibration isolating base body comprising a rubber elastic body forming a portion of a chamber wall for enclosing the liquid.
The following liquid enclosing type vibration isolating mount of a first type is known.
Normally, two metal fittings on the side of an engine and on the side of a body frame are coupled via an elastic base body comprising a ring-like rubber elastic body, a partition and a diaphragm comprising a rubber membrane attached to the side of one of the metal fittings in a sealed state. A first liquid chamber where the vibration isolating base body constitutes a portion of a chamber wall, and a second chamber where a diaphragm constitutes a portion of a chamber wall, are partitioned by the partition and the chambers are communicated with each other by an orifice in the partition by which the vibration attenuating function and the vibration isolating function are achieved by a liquid flow effect of the chambers owing to the orifice and a vibration isolating effect of the vibration isolating base body.
According to the liquid enclosing type vibration isolating mount, there has been proposed a mount in which a high attenuating effect is achieved in respect to a large amplitude of vibration and in the case of a small amplitude of vibration at a high frequency region. To effect a reduction in muffled sound or transmitting sound or the like, a rubber membrane is provided to the metal fitting on the side of the first liquid chamber where the vibration isolating body constitutes a portion of the wall chamber, so as to form a vacant chamber communicating with outside air (for example, Japanese Unexamined Utility Model Publication No. JU-A-4-97136) (refer to FIG. 18).
However, the proposed rubber membrane is formed in a simple flat plate shape, constituting a wall face flush with the metal fitting in respect of the first liquid chamber, a region effective in reducing the dynamic spring constant is narrow since the surface area is small and a resonance phenomenon of the liquid in the high frequency region occurs at a comparatively low frequency region (normally around 350 Hz). Therefore, the dynamic spring constant is increased in the frequency region by which transmittance of noise is increased.
Especially, although a more or less effect is achieved in reducing the dynamic spring constant up to around 400 Hz, the effect of reducing the dynamic spring constant is extremely lowered in the high frequency region exceeding 400 Hz and muffled sound at high-speed running is conspicuous (refer to Comparative Example 1 in FIG. 10).
A liquid enclosing type vibration isolating mount of a second type is known. There is provided a liquid enclosing type vibration isolating mount of a dual chamber type, for example, as illustrated in FIG. 19, where a vibration isolating base body 253, comprising a rubber elastic body, is attached to one opening of a metal cylinder 252 and a diaphragm 254, comprising a rubber membrane, and a partition 255 are attached to the other opening thereof respectively in a sealed state. An inner chamber of the metal cylinder 252 is partitioned into a first liquid chamber, where the vibration isolating base body 253 constitutes a portion of the chamber wall, and a second liquid chamber 258 where the diaphragm 254 constitutes a portion of the chamber wall as does the partition 255 where both liquid chambers are communicated with each other by an orifice 256 provided in the partition 255 whereby the vibration attenuating function and the vibration isolating function are achieved by a liquid flow effect of the orifice and a vibration isolating effect of the vibration isolating base body. Numeral 251 designates a main body metal fitting in a cylindrical shape and numeral 259 designates a metal fitting for fixing.
With respect to the frequency characteristic of such a vibration isolating mount, as illustrated in Comparative Example 2 in FIG. 17, the dynamic spring constant is high as a whole and engine noise, such as muffled sound or transmitting sound or the like, of an engine at high-speed running is significant.
Further, with respect to the above-described liquid enclosing type vibration isolating mount of a dual chamber type, it has been conceived that in order to reduce the engine noise, a rubber membrane 260 in a flat plate shape is provided at the central portion of the partition 255 as shown by FIG. 20. However, the region effective in reducing the dynamic spring constant is narrow and since the surface area of the rubber membrane 260 is small, resonance phenomenon of a liquid at a high frequency region occurs at a comparatively low frequency region (normally around 350 Hz), the dynamic spring constant in the vicinity of 350 Hz is increased, as shown by Comparative Example 3 of FIG. 17, resulting in the transmittance of noise of an engine, or the like, being increased in this frequency region.
Also, Japanese Unexamined Utility Model Publication No. JU-A-4-97136 proposes to provide a rubber membrane on the side of the first liquid chamber where the vibration isolating base body constitutes a portion of the chamber wall so as to form a vacant chamber communicating with outside air in a liquid enclosing type vibration isolating mount of the dual chambers type. However, problems similar to those in the vibration isolating mount in FIG. 20 result.
The present invention has been carried out in view of the above-described problems and it is an object of the present invention to provide a liquid enclosing type vibration isolating mount capable of significantly reducing the dynamic spring constant over a wide frequency range, which is excellent for reducing muffled sound or transmitted sound when an automobile or the like runs at a high speed.